kms: Return a more specific change if there are no devices

In case we have no devices, after a KMS update (both because they've
all have been removed or because there were none), we may need to behave
differently compared to the case in which nothing changed, so add a more
specific KMS update change type

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1964>

src/backends/native/meta-kms-types.h

@@ -68,6 +68,7 @@ typedef enum _MetaKmsUpdateChanges
 {
   META_KMS_UPDATE_CHANGE_NONE = 0,
   META_KMS_UPDATE_CHANGE_GAMMA = 1 << 0,
+  META_KMS_UPDATE_CHANGE_NO_DEVICES = 1 << 1,
   META_KMS_UPDATE_CHANGE_FULL = -1,
 } MetaKmsUpdateChanges;
 

src/backends/native/meta-kms.c

@@ -576,6 +576,9 @@ meta_kms_update_states_in_impl (MetaKms  *kms)
 
   meta_assert_in_kms_impl (kms);
 
+  if (!kms->devices)
+    return META_KMS_UPDATE_CHANGE_NO_DEVICES;
+
   for (l = kms->devices; l; l = l->next)
     {
       MetaKmsDevice *kms_device = META_KMS_DEVICE (l->data);